1 Field of the Invention
The present invention relates to a near-field light generating device generating near-field light and an optically-assisted magnetic recording device including the same.
2 Description of the Related Art
Owing to the recent progress of the information society, much attention has been paid to mass storage systems such as hard disc drives which are magnetic recording reproduction systems and optical disc drives which are optical recording reproduction systems.
A magnetic recording reproduction system records information by means of a direction of magnetization of a magnetic bit which is the minimum unit of recording on a recording medium. Provided that the magnetic anisotropic energy of the magnetic material is Ku and the volume of the magnetic material is V, the magnetic energy required for the magnetization reversal of a magnetic bit is represented by Ku·V. In other words, the higher the magnetic energy Ku·V is, the higher the energy required for the magnetization reversal of a magnetic bit is.
As the recording density is increased in order to increase the capacity of a magnetic recording reproduction system, the volume V of a magnetic bit is decreased as a matter of course. In this regard, assuming that two systems adopt materials having the same magnetic anisotropic energy Ku, the magnetic energy Ku·V of the magnetic bit in one of the systems having a smaller volume is small as compared to the other system. In other words, as the recording density of the magnetic recording reproduction system increases, the magnetic energy of the magnetic bit decreases. This induces the problem of super-paramagnetism which is a phenomenon in which the magnetization of the magnetic bit is reversed due to the heat energy at around room temperatures. When the magnetization of the magnetic bit is reversed, the magnetic information of the magnetic bit varies even at room temperatures, with the result that the magnetic recording reproduction system cannot retain the recorded information.
To avoid this super-paramagnetism which is associated with magnetic bits with small volume, it is preferable to use a material with high magnetic anisotropic energy Ku. Using a material with high magnetic anisotropic energy Ku, the magnetic energy Ku·V is large even when the volume of the magnetic bit is small, and hence the problem of super-paramagnetism is avoided. However, this scheme is disadvantageous in a different way in that a material with high magnetic anisotropic energy Ku requires a stronger magnetic field (coersivity) for the recording, i.e. for the magnetization reversal of the magnetic bit.
A technology devised for solving this problem is an optically-assisted magnetic recording technology. The optically-assisted magnetic recording technology is arranged as follows: For magnetic recording by magnetic bits, a magnetic bit is heated by condensed light so that the coersivity of the recording medium is lowered. In this state, recording is carried out by applying a magnetic field to the magnetic bit. As the temperature is lowered after the magnetization reversal, the coersivity of the recording medium is increased. In this way, it is possible to prevent the super-paramagnetism in which the magnetization of the magnetic bit is reversed by the heat energy at around room temperatures.
This optically-assisted magnetic recording technology, however, also has a problem. As the recording density of the magnetic recording reproduction system increases, the size of a magnetic bit whose magnetization is to be reversed is decreased and hence the radius of the magnetic bit heated by light must be shortened. In this regard, light has diffraction limit and hence the size of abeam spot cannot be smaller than its wavelength. For this reason the magnetic bit to be heated occupies an area larger than the wavelength of light. In this way, the wavelength of light imposes limitations on the possibility of the increase in the recording density of the magnetic recording reproduction system.
This problem associated with the optically-assisted magnetic recording may be solved by the use of near-field light. The near-field light is light generated around an object smaller than the wavelength of light or around an opening smaller than the wavelength of light, when the light is applied thereto, and this near-field light is localized within a range smaller than the wavelength of the light. Adopting this near-field light as a light source for heating the recording medium, it is possible to heat a magnetic bit in an area smaller than the wavelength of light and to realize the increase in the recording density.
To heat a magnetic bit in an area smaller than the wavelength of light by near-field light and to carry out recording by precisely applying a magnetic field to the magnetic bit, the point of generation of the near-field light is preferably as close as possible to the point of generation of the magnetic field.
Examples of a method for generating near-field light in an area smaller than the diffraction limit of light include a method in which a narrowed portion shorter in size than the wavelength of light is formed by a conductive film made of metal or the like and light is applied to the narrowed portion and a method in which light is applied to a metal scatterer smaller than the wavelength of light. In any of these methods, light with higher intensity than the incident light is generated in an area smaller than the wavelength of the light, by means of surface plasmon and local plasmon. When the aforesaid narrowed portion is used, an intense magnetic field is generated around the narrowed portion by directly supplying a current to the conductive film. When the aforesaid metal scatterer is used, a micro coil is formed by the conductive film to surround the scatterer and a current is supplied to the coil, with the result that a magnetic field is generated in the vicinity of the point of generation of the near-field light.
The light applied to the near-field light generating device is typically applied from a light emitting device outside the near-field light generating device. To efficiently generate near-field light from the near-field light generating device, it is necessary to align the light emitting device with the near-field light generating device with nanometer accuracy, which is equal to or shorter than the wavelength of the light. Taking this alignment into consideration, the light emitting device, the near-field light generating device, and the magnetic field generator are preferably formed integrally by means of semiconductor process or the like.
In a conventional optically-assisted magnetic recording device (disclosed in K. Hongo, T. Watanabe “Lensless Surface Plasmon Head with 1 Tbit/in.2 Recording Density” in Japanese Journal of Applied Physics Vol. 47, No. 7, 2008, pp. 6000-6006) in which a light emitting device, a near-field light generating device, and a magnetic field generator are integrally formed, the light emitting device is a semiconductor laser device, and the light emitted from the semiconductor laser device is directly applied to the near-field light generating device. A technology similar to this is also disclosed in the specification of U.S. Pat. No. 7,547,868.
Now, a method of generating near-field light will be discussed.
FIG. 19 and FIG. 20 are respectively a perspective view and a plan view of the optically-assisted magnetic recording device disclosed in Hongo et al. and Japanese Unexamined Patent Publication No. 2008-90939. As shown in FIG. 19, on a substrate 901 which is made of an optically transmissive material, a stick-shaped scatterer 902 made of a conductive metal is formed. Surface plasmon is excited in such a way that the longitudinal direction of the scatterer 902 is aligned with the polarization direction of the light and the longitudinal length of the scatterer 902 is suitably arranged in accordance with the condition of the excitement of surface plasmon.
To the scatterer 902 suitably disposed and arranged as above, light is applied on the lower face of the substrate 901. As a result, as shown in FIG. 19, on a light receiving surface 903 to which the incident light of the scatterer 902 is applied and on a light emitting surface 904 which is opposite to the surface 903, surface plasmon is generated on account of the localization of electric charges caused by an electric field of the incident light.
When the resonance wavelength of the surface plasmon matches the wavelength of the incident light, the light and the surface plasmon are coupled and surface plasmon occurs, and hence the scatterer 902 becomes an electric dipole which is strongly polarized in the polarization direction. In the state of electric dipole, large electromagnetic fields are generated in the vicinity of the both ends of the large scatterer 902 in the longitudinal direction, with the result that strong near-field light is generated. Although the distribution and intensity of the generated near-field light strongly depend on the structure of the scatterer 902, the near-field light on the surface intersecting the polarization direction of the incident light is typically more intense in a portion with a high curvature than in its surrounding areas, because of the concentration of the electric field in that portion.
For example, when a stick-shaped scatterer 902 of FIG. 20 receives light whose polarization direction (direction of electric field vector) is in parallel to the longitudinal direction (E1) of the scatterer 902, the concentration of electric field occurs at the apexes of the end portions 905 and 906 where the curvature radius is large, and hence intense near-field light is generated at these points. When the light whose polarization direction is in the direction (E2) orthogonal to the longitudinal direction of the scatterer 902 is applied to the scatterer 902, near-field light is generated at the edge portions 907 and 908 extending in the longitudinal direction of FIG. 20.
In Hongo et al. and Japanese Unexamined Patent Publication No. 2008-90939, the aforesaid scatterer 902 is used for generating near-field light and a magnetic field generator 909 is provided for generating a magnetic field. The magnetic field generator 909 is a micro coil provided to surround the scatterer 902. As an electric current is supplied to the micro coil, a magnetic field is generated in the vicinity of the scatterer 902.
As discussed above, it is possible to generate intense near-field light by means of surface plasmon resonance by using a metal conductive film. As noted above, the surface plasmon generated on the conductive film is extremely sensitive to the polarization direction of incident light and the irregularities (curvature radius) on the conductive film edge. For this reason, in case where unintended irregularities exist on the near-field light generating device and/or the applied light is polarized in an unintended direction, the electric field amplification by surface plasmon may occur in an unintended portion, and intense near-field light may be generated in that portion.
For example, as shown in FIG. 21A, when there are irregularities at the edges 907 and 908 of the scatterer 902, intense near-field light (denoted by A and B in FIG. 21B) is generated at the apex portions 905 and 906 in response to the light which is applied to the scatterer 902 and which includes a polarized light component in the direction of the arrow Ea in FIG. 21B. However, when light including a polarized light component in the direction of the arrow Eb in FIG. 21C is applied, intense near-field light is generated at the irregularities (denoted by C and D in FIG. 21C) on the edges 907 and 908. Near-field light is generated in unintended portions in this way.
The scatterer 902 is manufactured by a semiconductor process such as lithography. In the mass production, microfablication is typically done by photolithography, and the microfablication by photolithography involves irregularities of about from several nanometers to several tens of nanometers.
The optically-assisted magnetic recording technology of Hongo et al. adopts a semiconductor laser device as the source of light. The light emitted from the semiconductor laser device is directly applied to the scatterer. A typical semiconductor laser device is generally designed so that light which is polarized only either in the in-plane direction of the active layer of the semiconductor laser device or in the direction perpendicular to that in-plane direction is stably output by lasing. The polarization direction of the induced emission is typically controlled by controlling the crystal strain of the well layer of the quantum well structure formed in the active layer of the semiconductor laser device.